”WNW MEV’SJS :12 SEE! W3 EW‘JE‘E 4H ST . R J 'J’EJEFS g *9SMJ‘07 *Eg2mg1”:u.s 4E / '. 'rvuzxwan w "‘1 ~ EEDM :HESIé' .5 r 1 . Vat-«C7 .7 L. I" 5.. " ')9.\ ‘-< ‘ " ’ -\. I .J-' - 5‘ L:‘ .34 M 1 . c ’ - “"‘ ' .‘ L «‘ s. o JJ..K 3 ¢.‘ 1" r , '1" As . I .i ‘ .50‘ ‘ g'E ‘4‘ t"‘,’ . 4 a I‘., ‘ ' A '. 3 n ‘-. thl.iill‘;iJL-A l-lu.‘ ‘ A) I ~é1-j-b'lli b'b‘g f: . l‘. :‘f -'.‘2 -—_—- (Lx- E‘gtb M‘h ,a " H “y; ‘. H“ :Dc‘tlbt {1’}; 2' . 1:. - v i ‘1‘» .,‘J ,. h. ‘3 JV JHESIS \ In order to visualize were clearly the merits of both steel and concrete structural designs an attempt will be made to censure “3,110 materials in the wa"s in W”ich tIsey control the factors that lead to their sole cLion. Lany of the features that are characteristically allied Lo steel. permit its use in fields not in¢a1ed by concrete. to such uses of steel no further mention will be rude. Cnly to the uses of steel in3places where some considerable difficulty obtains in sttehpting t0 choosex hi all heterials are the best to use.w1ll this treatise be devoted. Good engineerirg'calls for the use of those meterials which, uncler 3ivcn cond:Lt:ions, will most eificiently and ecero 'czllr :e't the required conditions. Tllere is no particular material nor ary one type of con:struction that proves to be the best universally. The proper com- bination of materials structureall.y a‘2d otherwise requires diliuent and well directed study. The features to he :aneetQ ated in this thee is are time requirei fur s.nstruct103, la :ur prelzi 0.5, effect 01 weather, meintewunce, eufl cost of eunstruction. A ceusiaerable space will be ellowefi for the relating ‘- ef the richiwan State cighway hepartment ‘v-J wractice of the in its corstructlo of Lighweg Lr1u3es. lhc results of engerierce of cnitrtctors, sud the history or bridge dev- elepuert in fichi3an will be briefly sifihted as it affiects I the topic. 10341 L .,uch bridge that is LLL‘Qigil‘nti. preem ts {rowers that are c arectvrlqtiUhllv its rawn. 11050 prollens Lew he a coiiixatjan uf rupee e tul or sneerfler" factwrs of sec“ on orfier that shuther bridge lire it uihht rct he requir— ed for qqiLe a number cf yearn. ibis Pact clove vtint- tn the breai UuLhUP of ue«i~ns the departme t is rvqniref e havecrszu8.EJnfil only i=5 .Je lrluw\t'o.r: ~ 1 . J .- . .2. .. . .~ ‘ '3 ; '. ~ ~ ‘t 1 ‘I . ‘1 ~. 1 -1 1 1 r . -~1 1 x '1 .1 1 .12! (11 cup; ‘12..) 1:.“ L 'JL 6 1 3-351 ' . . - I’ -1. : . - ' - ». -. - . ~ A": 1 . ta- - '1""-'- n :—- - ..1 a ”-1. r r 'Vawaw no: concrete ore cw 11st“ w 4.00;.1c :su.; u t .v t1L» I 1 I - ' ‘ " - -_ .1 , ‘ . . . - -- . -Vr 101. \- 1- . , ) ‘f1r \ .1) . 1rd..- 13 _ {I ‘(\ Iii-£11.}; 1;. D§ - I 3.. a a -.J. .03.. in 3 (ll ‘JL‘Iujt1 b4 .Lr ".a ifiv , 1.1 .M- - .. 1.. , 1 . .. ~. ,- ‘ .., ”1.... 11"- ...,. 1. “,‘4 , -° .2 ... {a l V \ I ‘ x ;J ’\ 1- -. 'J at b v ' ov- f... L! I 5:, ..J :. g - _ J ‘1 l- K‘ ’ .. 1. -.’. . c U ‘. ‘LU .. , 1 .. . . . ..2 . ' , 7 _ s , ‘ . . —. Cfflih ('31:; t1", 1'41 11- U.'(' 3:9.‘01‘ p -. ._t '3. ..'_‘ 1'31. {1 7 5C [’16.- fl' 1! -' ‘ ‘- ‘ -- c n (1'1 1.1. :\-" .t- ' i I;.- ' "r " " .\ ."91'0 '1 11.1. , -1611. .1 11 1... .1». -11 1 1. 1.1 -1~ . 1.111. 1..» 1. - .L: :11 1 1 - —,4 .1“, “z ' 1-1 . 1. 1~ 1... 1 11 t ' ' 1 -=. . LN, :-I"I'cr1- u'f 13--.! 11 ‘)l LL-,J(- L“ ..1 11.....A. a - 18' . O ’. 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' '1' a ‘J r 9'; j‘11"1 ‘ -\'- {I 2 . ..— H J 1‘6 .L ’ “::’.: b ‘ I ”1 ~ 11. \ ’1 \ '“ 1 +1154 1 L '1. .\ 1 »' ’ -' ' "1 ‘1 1:- . 1 ,- ‘J 21-...” O -1. .1 ¢ ..LL. 2 .‘.L .f . (a; ‘ * .va .1' f 11‘ ‘1 ’ “’11,: - .. \I a. K , .-_ ‘ .. .. __ ._ .1. ‘ . .- 44-1 , : I ‘1.- . -11 _‘ . . 1, . -.-‘ 2.1. 1.1.. 1.1 .111 .J. (‘11; {51’ :b._-. .- ‘ Lug-1...}... - ‘.‘ '- ' . * '-' 1'- . , . '. I .. ,. .9 .. , . -. . .z a... . _ ” . 1.. 11- 14‘} s- . 1.1 (I . 11 . U 1- Js‘. 1.; ‘- a. .Li ..1. u ’1‘.‘-'« ' ’. {.17 ., ' - _ 1 '. . . “ "1 ‘0 . . -' '1 - 1 . . ‘I -v - s - "l‘ ‘ .4 u . . ~- ,_ '4 .a .--- J- . ' . U'.’ (to {1-1 tab J. 1‘ UJéx bu‘.JC-L :31?1.“_\v ‘2; U]. ‘1‘lt‘li...— \l’o.‘ 53 . _: g, -1- 7‘_- [1- 3 . _ '1' ,I 1... .'_' ,‘, ' .».én- , ,‘I -1 . , . -1 ,. .1 , r t {11.1- .' 119.1. .0 1...}: 1:131. 111‘) .1... (.1 ..; Le CEHLLCL 7331"" .l”-$l‘ ' 1 «‘15. - 1).:- '.‘-~ '° ‘11-‘51}: n." ‘1' 1- L- .--.,-' 7.. '. -. 1 fl -‘ C... 1.1 .111.” J- 111 1111.111..- 1.1.1. 1.1-1.: ¢ 131?. -111.:..1.-‘; 1.31).“ 110.11.? J V‘. 1. 1 1 . .. ..J- -. . , ...... 11. 4- 1.1, .1. 1 . - - “'1. .- - -- -- . 11 1 ‘1 . a '1) 1. . ~.-‘ 0 v . 31D ‘1‘!) -'J .1- 3 - 1 1: HE'HC 1;!4‘1 -1 ll't , J I 5- I; ' L’) .J'l. 1.." o 1 JO 1.1.9.1, ‘(3 1.. ~ r . ~ ‘- + 1- ‘w 1.1 r, . . 4".\1'. 1’ '- ~- -- o'o ‘1 '1 9 '1 1:11 1 11 ”1.2.. 1 , " 11 at: it 131: 1 1.7.1.91" 21......“ .1. 111‘ ~21 ‘ 13". .1111 ‘ 1.1 “ . 1 ‘1 . 1 '1 . . 1' . .n .- ..I. . '- . . . . . I . , (‘. -* 1 - 1 . .7 1 .' .5 1 ( 1 .. 1 1;- 1; 1 1 ... - I ‘uikl 3; 4‘..." I 3 o... L‘ ’.'-"4 1. ~. J. :‘u‘ (> d. - ‘AJ‘ 1. -I -I.’ stlj'btqi I ' u. ‘df.’ .' ." . 1 ‘ ." J‘i .' 1 . ‘ 1 . n t a h .I n v '1 1. 1 . 1 - 1 ‘ 1 - - 1 » Del- L1 103 w.: L.1Le Hr 1.» couclcte mt V43 r1aced I ~ ' ~ ' '9 ' ‘ n ‘ ‘L 'D ' ' .1 '7: -~. . .. “I ‘ ,1. , n . .. u . ,1 ,-.1_' p‘ {3‘ g‘yoh‘b, (‘1 t; 1.1- . ‘1 . .. ..- ° w 3 -' -.. . . . n . - ‘ I“ ,«1 . - ‘11 ‘ ff _ ~r v V ‘- .n ‘f- 1 ,\ q ~ ~ 1.1 cu. 81.6.... .....1-e 1.1-1.111” 2., <11. 12:, 11:11. 1‘ (1... -. A further saving, is also Clue to the fact th wt t~ie bridge seat elevation is dew 3' for the cone rot e supe~etruct- ure,the elHut ent tblcnn.du veins 2' while for the briége seat elevetionin the casein? steel superstructure which is 5' a thickness f 5' is required. At the top of the footing the concrete abutéent thic: :ness would be less for concrete superstructure than Bar steel. This is due to the fact that the presaure of the backfill to be re- sisu “ly the ebutHent is less in the case cf concrete superstructures, since all backfill pressures are above the bridge Beet. That is, for the upper 9' of fill are cam ied direct 3* through the con mete snpers tructure, whereas for the steel the abut.e“ts thense Ives r st res- ist yressure frnn this 9' fill. From the above facts we see that in the substructure a considerable saving was effected when the desigv for steel was superseeded by that of the reinforced concrete. A comparative estimate of tests with steel truss super— structure and with reinforced concrete girder has been V preparefl and may be fauna an the followirg page. It will be note& that the superstructure of concrete is slightly lore expensive than steel, however several facts have not been taken into cons idur‘fton and w: ill be d.i m4ed at this time. One of the important factors the bridge engineer takes into consideration is the matter of rigidity. Some of the oldrr types of steel D'iubvfi are war; poor exauplcs of rigid structures. With the concrete supc"structure as J 13..¢::;:;'.cnm in Lila awe, special 01.12.131’12515 mm ...;'-;i(‘. in the design wiL12rc5urd to this fgctur. In a rigid structure with large inertia the turdcncy is for tLo structure to abs orb 01-0 impa ct uLd live loans with but small uvcrstr- “336% in tnc mergers. Conflarative tlxatbu of Bogus witL P(inOPCP d cor- creto girder superstructure and with steel truss super- strucburu-- lat-5' spun. :EIU' roadway. Concrete suvwcr tructurc and concrete abutrents. AbutLentsl Lurp sum $Su74.11 211 Lu. gas. hraJe O concrete. ‘ilgjug '339u.{nn 3U}( rstructrle. 370 (311.3113. grade A N $1.0. 00-413.109.00 Huanfifl re-Lars 0 .US---——---—- 3145.00 3w$uUf ”tru. steel L .16------- 484b.LU luli:h.‘u0 7" ‘_‘ Cemerzt finis.heJ abntuent.3 b:u. : DUQCI‘Sui ‘LlCudI‘t, 1 i;:‘...’. :13 Engineering mad sneervision 2th..2 10631 estimate of first cost-----—-831a;3.80 anrual maintenance. Annual erreciatiun. r”: n- . 1 AOL, L‘- Si\ .0)‘.‘ . . . 5"“. ‘ _ n L, _. ' Lila-lit (4:45. (or! fl "'-4_—- A J57}. ' 09" ’71.." '-- 1"- ..-. ...--J-. 1....9! .. {‘rsgv’a-w ,., 1...: uilJQtCL‘. it; {Lil CU'IU .LU §;Q-:L'i (it‘ll. bJ-?Q‘_;}£, 5 ..J"; Stecl sunprstrncturn zwfi cancrnto abnt ants. fihutments $5740.8U’ Piling , 2390.00 Steel superstructtre 1%:(3'hL‘IL C; .‘I'Ub 1'11:3"OOUO Co TOPGtQ flcnr 8305 cu. Fdfiofl 5;4£)ILU 1957. abut}: ...HtS 454).).1C) l 1001' 14,1000 ll“: re-hilrf; C, 9')? SL505“ anineoring and supervision 28L0.00 Field gairting ““335.09 Total esti: ate of first coqt-—--9~ £73.06 Annual maintannhcc 110.06 Annual depr.——~BG x 26" 2.60 53;.54 Capitalized at 1[ 675.54 Estimated total cost in perpetuity $db165,43 ¢&e:~--;: }-2:.'irifiééiét?£?€?£$¢5£$€$€9£?€i€5¢$ A still furt’ or saving could have hnnn oftvcbed by having; a 5111.310 64..)mt1'4146ttw'. in t:' 20 abs‘rm jab 11' tho stool supergtrncture Dal b0”u uJOx it WUI-d' Juv:: necessitated letting at lncst two contracts and perhaps more. Such work as suhstructurc of re: 3 reed ca2creta would be let to a separzztc concern whiie that for fatricatiofi and err- ectiou of steel tn anuthvr 020 LP two on tractors. ibis could '52:: .1-433' 2 11:19:12.: VCl‘j" nicely 123,- lettiag the contra- ct in one and obtain lowar unit prions. The price quote? for tflc steel anyorstructuro is typ- ical of tru:3ses or"cctntlr1thout field paint and this of course must be tafien cart of at an extra cortract. The engineering sugarvi"inn for the steel structure is sligFt- 1y higher than for the curcrcte structure. In the constr- uction of the steel structure three contracts are requir- ed - atutuents - superstructure- floor. Each requires a dif?erent inspection. in adaitiou to this a shop inspect- ion is required on all structural steel.1t will be noted that the first cost is in favor of the steel structure. Several other items, however, rust be taken into account. First, the depreciation allowance on concrcte is 1% annu- ally as compared with 25 for the steel. The concrete str- ucture will require no maintenance exrenditure while structural steel will require an annual average mainten- ance of $110. These surs capitalized at 4% the resulting total costs are corsiderarly in favor of the concrete. Inaddition to tris cocnarative data two sets of curves are -u'jjJLE; stah'zibtfid. ”BEBE: 1. shows the actual cost Of the structural steel for swans ranging from 70' to I75' inclusive for a total of tbirteen jols. This curve of unit cost represents the price for steel errected and painted, exclusive of concrete floors. Sheet 2 illustra- tes the price per cubic yerl for concrete superstructur- es in place, he data being plotted for lfi flifFerent Jobs, Sheet 3 represents the quartities of materials required for concrete girder superstructures in spans from 35' to 90' inclusive. Also the quantities of structural steel inthe truss structures free 73' to 175' inclusive are shown. The number of cubic yards of concrete in the con- cretc floors is likewise included. This sheet 3 in giv- ing quantities has been niettefl in such a way that it La? be used for illustrating any particular problem that night be selected. This has been arriveJ at by dividing the total yardage of concrete in the concrete girders by the number of square feet of the resulting roadway} also the concrete floor yarfiagec are in cubic yards per square foot of raceway. The individual circles platted for steel giving the bridge numbers from which actual widths W’Pe tench; The cancrete girder curve is platted from actual drafting roow records. Sheet 2 is platted for maximum concrete girder span of 35'. There is a very material saving or reduction in unit costs in urorter spans and in order to prove the rr511“t u , the anxinux span is usel. There can be no ques» tion of the fleet that if cconory can be shown‘on a 90' spam with concrete 3 far yreater economy exists for short- er spars. Sheet 2 shows tfiat for all Jobs on which a lung sum is bid the average price per cubic yard for superstructure; arrived at by taking the average price of the Job an} auuing $7. to it. This is a liberally h1g2 estimatd.for the 0031 of the superstructure and free the resultant platteu pnints on sheet 2 the straight line represents a high cum conservative price per cubic yard of superstructure co crete. 32‘ .NNQN NRQN fiNfiN .QNQN .3 EV 63¢. I to: Ill “I '0' ‘N v“ ‘0 5‘0 05 K M04 480’ 9/03 9 - /90{S’ [llh/DflJ/Q §' N (385% 3:5.» «oh 13% .. o .g 8“ Qhkbhanw 530.3 Nahum. «\thbbbflm. gm k03w$3h .Nakhhx‘m. a; s seen 364 . ass 33 c3». 32 g3» » 3 [9 ”W99 if 0//‘0" 8 $370.: 431/ N quausfi #5thth .8 .QQQQQ hosts.“ .N. Ik “M3 . . . . . _ _ .. ....... (.4... «A RINHK >\\ \an‘u. , _ .. . _ . S... ”s ..3... . 5*. manW“ 3s 3.. e3 34 e! 34 3s 3 8... 8 es 3 8 am... - . w V. M, 0‘ _ 3 , . ‘gav . Is ' . Q W67 1 3 0.9 8 15' l ‘. 85W ' «w. 8‘ 1‘ .30 . 5.x 0.1 \ m K —_a 9. R . wa-WWQ 4nal—WJ a 8 ‘7 l _3 10:13.17? 1‘ 3 l l. {50.49” {)4} 6.9 19d ”kW-4”!" / 1 -l - 5mm {a a! 3: Rs. ”—1“ L__§ f mm ”1 . . . . A .QKBV thshsé 39:.ng ‘9‘ \9 .‘K .k i 5\\§\v .J‘g . . P ~&.°\l‘ .EQ‘oQ . ... . WM. ‘3. up... 2.6. .. . . .. _ . . afi‘KQ .993..\K... \UW‘Q . - 2.. . ._ .. _ . . .. i. . .. awn in...» . _ . We. .. . .. 15K.§\\Q . ggfig . .... .. . - ..... m .1. . .. . . .. .. .. . . . . ..uanm klflgjr For a 90' PP“" the quantities involved are 1.4 cu. yds. of concrete per sq. ft. of roadway for the concr- ete girder, 71% of structural steel pvr sq. ft. of roadq way for steel superstructure and .02 cu. yds. of concrete per sq. ft. of floor surface for the concrete floor. Then if '1' represents the price in cents per cu. yd. of superstructure concrete and 'y” equals the price of structural steel errected and painted, for a on' girder, then 3 equals “08, for equal costs. This means that for equal spans with structural steel at 5 cents per 1b., the concrete girder Light average 30 dollars per cu. yd., at 6 cents per 1b. the concrete girder night average $36 per cu. yd., at 7 cents per lb. the concrete girner might be $43.50 per cu. yd., at 8 cents it might be uhl.TP per cu. yd.. at 9 cents $34.50., and for 10 cents it would be quoted at $u0.uu per cu. yd. For a 75' open similiar figures vould.be as follows! Steel per lb. 5¢’ concrete per cu. yd.-$3l.0o. o¢--$4U.Uu. 7¢~$d7.00. 8¢23$s4.50. s¢..$61.?o. and for 50¢..fiou.oe. Take an exarple as of Sept.1, £033. sheets 1 and 2, The price per lb. for structural steel as shown on the curve is 8.U¢ For a 90' girder therefore you could aff- ord to pay $52.19 per cu. yd. for superstructure. Referr- ing to sheet 2 it will be noted that actually you could pay $43.00 . This means that if you would build a 90' span of steel, it would coat 22.0f more than to build it of concrete. Going back to Jan.1, 1923, steel prices are 7.1fi per 1b., for which in a 00' girder you could afford to pay $43.10 per cu. yd. for the concrete. From curve 2 we find the actual cost to be $40.50, thus choice would be again in favor of the concrete. how take the low price of steel in 1931 and it will be noted that one could afford to pay $33.00 per cu. yd. for the concrete superstructure. This price prevailed through the year £021 and corresponding prices for the concrete ranged from $33.0 to $33.00. This is on the assumption, however, that prices for concrete superstructure are $7.00 per cu. yd. higher than the average price per cu. yd. for the Job. This is considered a very conservative allowance. The above discussion is based on the ratio of first cost of actual superstructure. In addition to the actual saving on first cost due to superstructure there is a saving in the substructure for concrete girders as again- st steel bridges, which varies with the height of the abutment. This was found to be in the specific example which was discussed near the beginning of this article. he top 8' of the abutment, that is, from bridge seat to roadway is dispensed with in the concrete girder since the girder itself serves to retain this fill. For a steel structure the abutnent must be carried to full height of roadway in addition to a very thick section. In addition to the first cost of structures the matter‘ of maintenance must becausidered. Assuming that a steel structure will require a coat of paint every three years and that it will take .5 gallons of paint per ton of steel, the cost would be $1.75 per ton of material. Labor usually runs from 1.5 to 2 times the cost of material, thus making, an approximate cost of $5.00 per ton of steel or .25¢/lb. Then assuming an average price per lb. of .6¢/lb. for steel this gives a three year maintenance charge of about 4% for the three year period. In addition to the first cost and maintenance charges in conparing ‘omparative) relative values of the two types of structures, attention must be called to the ability of concrete structures to resist increased live loads. For example the 90' girder has .142 cu. yds. of Iconcrete per sq. ft. and weighs 575 pounds. This carries a live load of 100# in accordance with the specifications of design, making a gross load of 675 lbs. per sq. ft. If we now increase the live load or weight of truck using the highways by 50%.thermfitéribi.inrtfie4girder would be overstressed merely 50%rof 675 lbs. or 7.5% lovecer in this interval the concrete will have increas- ed in strength more than the increase in live load, since the design for concrete is based on 28 days strength and a result in gain in strength from a 28 day to a 12 months period will easily reach 20%»for the material. has had a structural steel design been used, it would take 71 lbs. of structural steel and 80 lbs of concrete floor or a total of 151 lbs. per sq. ft., carrying a load of Q99# per sq. ft. or a total load of 251% per sq. ft. If with this structure the live load is increa- sed 50%.the stresses would.be exceeded by an amount equal to 507. of zoo-3%, or 20% as compared with 7.4% for the concrete design. In steel there is no gain in strength but rather a loss due to age. In the matter of durability, no one can say what the life of good concrete is. No experienced person will att- empt to say, but any concrete engineer will admit, the life of a concrete structure is as long as the life of a steel structure other things being equal. The matter 0 of the life of a structure in this day aha age of rapid changes is not the determining factor since the incre- ases in width of roadway, load capacities,-and the whim of thepublic in regards to appearances and aesthetic treatment are much more vital features and imply that most of our structures will be removed.before they are worn out. The design of reinforced bridges fer 70' to 90' span have curved top chords and bottom chord brackets. The first 90' girder span was completed at Tecumseh, Mich. Now some notably long spans and difficult structures are to be seen under construction. The state has on s several occasions tested some 90' girders and found then exceptionally stiff andreuarkably free from impact and and vibration. The unit stresses are well below those for which the structure was designed. The department of bridges is experimenting with long spans and expects to replace 100' to 150' steel spans with concrete trusses. This tends to obviates the difficulties and long time contracts resulting from the handling of substructure, superstructure, and floor contracts and still keep with- in the limits of true economy. The advantages of having a single contractor, using common and readily securable materials, and the resulting rigidity and large inertia with consequent ability to abiorb impact and live load increases with but small overstresses are highly des- irable. The policy of the state of Michigan tends largely toward concrete design. Bractically 90% of the bridge bridge construction in that state being of that type. It has been there found profitable to use concrete up to a point where the first cost of the concrete may be 10% higher than the first cost of steel, due to the fact that maintenance of steel is high and also annual depr- eciation is greater on the steel structure.During the past two years the bridge department has eliminated a great percentage of the steel bridges formerely in use by extending the concrete girder design to and includ- ing 90‘ spans. Now practically no consideration is given to steel construction below 100' spans. However steel bridges are not without argueflents for their use entirely. A few of the arguements hat seen to be in favor of steel as a bridge superstructure nay be found in the following aragraphs obtained free consult- atiors with men interestud in the construction of steel bridges as a profession. The concensus of opinion seeued to be in favor of con- crete construction for beauty and aesthetic treatnent but this point is not one of consido~able importance. The points which should control the general design of a bridge as most folks seemed to agree were: 1. Physical conditions of the sight. 3. Requirements of the traffic. .3. ncquirerents of the purchaser regarding type, ornamentation, and special treatment. 4. honey available. It is maintained that a steel superstructure can be put on a cheaper foundation than can a concrete super- structure. The type of abutment and footing designed by the state department are not the most economical for the service required of ther.in naintaining a steel super— structure. In cases where a bridge is being built over a sink Tole, or in places where the soil is unstable and uncertain a steel superstructure is more certain to prove satisfactory because a slight settlehont, if one should occur, would do no harm t‘ the steel superstructure where- as it u ght ruin a concrete design. Types of bridges are being changed continually to root new require:ents. In this connection it is to be noted that of a total of 35o bridges put under contract to date 145, or about 40.55, have been of 18' roadway. Very few bridges are now being built with 15' roadways. Practically all of these bridges mentioned are less than ten years old but still many are being torn out in favor of the 34' or other width roadways. This tends to show that it is unnecessarily wise to spend large sums of p public money to provide for the future 75 or 100 years hence. no one knows what traffic con ltions will be then. {p U) the present time practically all of the hidhwny bridges have been constructed with roadways of 24' or less. It is rumored that within two weeks previous to the pre- sent writing he covernor of the state of Iichigan direct- ed the bridge engineer for the state to start changing all bridge plans and provide for roadways of from 30 to 30 feet on all future bridges. This move will make obsolete all of the exnensive massive structures designed by.thc state so far as state work is concerned. “he Lnows how long these new structures will adequately serve the public need. “by, then, try to pwovide for a distant, uncertain future when chances are large that the structure will never serve over half of its contemplated life. In file cawe of steel bridges, when it is desirable to relocate a stretch of road, asis often done,and a steel briage is located on the ahanuoned portion of the road it can easily be noveu to the new bridge site or sozewhere else and put to use. in the case of a concrete bridge it would have to be abandoned entirely or else rnaoved at a considerable expense. A steel structure over a drain or river can be remover free thc abutnents and permit a drouge to paws through if the bed is ever cleaned out or deepened. Also the guard rails on a bean bridge can be removed to allow wide obleots such as houees, barns, etc to vase over when being uoved. in regard to steel bridge work not being satisfactor- ially done it is certain that the states engineers are on the job with the instructions to reject any part or all or the work if it is not preperly done. All finished work oust have their approval before any money can be collected on the Job. As a matter of unsettled arguenent, which can only be settled by reference to the records, it is maintained that the suhetructure, superstructure, and floor are all usually let in one contract whether the job calls for concrete or steel superstructure. The matter of sun-contract is optional with the purchaser hence the argument that delays due to held—Ups on one or more contracts seems Weak. «J. At present it is the policy of most organizet-ors thet are purchasing bridge structures to investigate what a reurby railroad has done over the sane streer near by in regards to length of span and clearance. This is done to get the advantage of the experience of the railroads. Their engineers are regarded as among the best in the country. yearly all of the bridges Within a reasonable length of span that are uilt by the rail- roads are of the plate girder type. If the railroads fire the plate girder superstructures will stand the vibrations and impact, and can give satisfactory and BCODOLic service, wouldnvt they be worth consideration for use on the highways? With the prices that prevail in Sichigan in $020 plate girder highway bridges can be sold, errected and sainted, for S¢ per pound. A plate girder highway bridge may not present as pleasant a View as a concrete girder; but in be light of economy and good business it is hard to believe that the aesthetic treatrert is worth the difference in cost, difference in the length of time that the road is closed during con- struction, ease with which the exact status of the bridge can be deteruined, and simplicity of cvnstruction. In conclusion the author wishes to state that a com- parative analysis of steel and concrete in structures made today would be of little use 10 years from toiey, rd. because of the ever changing rat 0 of prices, :et. 2oes of handling and transporting materials, and cbarges in the denaeds made upon the structure in its :ses. For ex:iwyole:A brridhe bu.llt durirg tl:e w:zr was considered best when it required he lee't l ibor and time for constr- 2ction but which would anve pronise of servirg for a roosoreble length of tire. Today that'bridgo is besst which presents the the most pleasing view when seen in the surroundings in which it is to be co structed. In other words aesthetic treotrent seems to have the greate. t bearirg on the type of brings chosen. Vhat features will control the d.e"i~n of ‘Wri :;es in years from today noone can tell. 1320 writer co: .2.~2e::1ce>i his i11ves~2tiggetioos for this thesis with a prejudice favoring steel superstructures fdr use on bridges on be public highways. This pre- judice was the result of a lock of knowledge of the adaptability of the concrete superstructure when used u22der the varying cendlti.ons tizot a bridge for stard- nrd use would be az7m1 lied. After concluding what was cossidere i on izport.iel inv-stigst;ion t}. e writer has concluded that for highway bridges that must continually bear increased loadings and serve the greatest possible length of time a concrete girder is the nest economic- al type up to and including spans of 100' in length. Enough data is not available to make an accurate com- parison of spans of greater length. Rough estimates indicate, however, and the writer is inclined to agree, that for the longer spans steel structures are the most economical. Data for this thesis was taken from a survey of the records of the bridge department of the State of Hich- igan, advice of friends, and from consultations with contractors, steel manufacturers, and others interested in the subject. my $14 % MICHIGAN STATE UNIVERSITY LIBR H H lllllllllllllll 3 1293 03178 3412 W“